I am a newcomer to the company, and I am responsible for informing others of the company's activities. What should I do? By text message? Telephone or ...

Newcomers should be modest. If the people to be notified are not too far away, it is best to call one by one if time permits. If the company is big, call them, but if time permits, you should inform the main leader to show that you respect him.

Don't inform you by SMS. First, you can't confirm whether he received it in time. Second, it is disrespect for people. Moreover, if he doesn't come to the meeting for some reason, he may argue on the pretext that he didn't receive the text message.

And even less want to use offline notification. If offline notification is not available, the responsibility is entirely yours.

Telephone notification is reasonable because it is the normal business of the company.